1. Field of the Invention
The present invention relates to an oil pump. More specifically, a plurality of pump chambers disposed substantially circumferentially in the oil pump under the present invention rotate in accordance with a drive shaft, with a capacity of each of the pump chambers increased substantially sequentially in a first certain angle range and decreased substantially sequentially in a second (remaining) certain angle range.
2. Description of the Related Art
An oil pump is known for a cavitation which may be caused at an intake step with an intake resistance increased. Thereby, the oil pump is ordinarily provided with a flow rate control valve on a discharge side, instead of a narrowed portion (controlling the flow rate) on an intake side, thus returning an excessive oil from the flow rate control valve to the intake side.
With the oil pump having the above system, however, a once-compressed oil is to be partly returned to the intake side without making any operation (function), resulting in a great dynamic loss of the oil pump. For reducing the returned oil from the discharge side, the following is taken into account:
The flow rate is to be controlled on the intake side. In this case, a bubble which may be caused by the possible cavitation is to be smashed slowly and mildly, so as to prevent a failure including an abnormal noise and the like due to the cavitation.
Described hereinafter are related arts disclosing an oil pump for solving the above inconvenience.
Japanese Patent Examined Publication No. 3074047 (=JP3074047B2, equivalent of Patent Number JP5099162) discloses an oil pump (referred to as xe2x80x9cINSCRIBED GEAR PUMP in its English abstract). The oil pump is of a trochoid type which substantially sequentially increases a capacity of a pump chamber in a first certain angle range and substantially sequentially reduces the capacity of the pump chamber in a second certain angle range (remaining angle range). An intake port is so formed as to be open substantially in an entire decrease range where the pump chamber moves from a maximum capacity position to a minimum capacity position. A discharge port is so formed as to be open in an increase range where the pump chamber moves from the minimum capacity position to the maximum capacity position, with the discharge port biased to the minimum capacity position. A wall portion of a casing defines a certain section between the maximum capacity position""s side of the intake port and the maximum capacity position""s side of the discharge port, which section is formed with a seal land portion for sealing a communication between the intake port and the discharge port via the pump chamber. The seal land portion is provided with a check valve. The check valve may open and close a bypass path which communicates the plurality of the pump chambers {facing the seal land portion} with the discharge port. The check valve has a valve body shaped substantially into a plate. The valve body of the check valve has a first side facing the plurality of the pump chambers and a second side biased by means of a spring (biasing means). A pressure of the discharge port is to be applied to the second side of the valve body of the check valve via the bypass path.
When the cavitation is caused in an intake range at a high speed and the like, the capacity of the pump chamber at the seal land portion may be substantially sequentially reduced with the bypass path closed by means of the check valve. As a result, the bubble caused by the cavitation is smashed slowly and mildly.
Moreover, in a state that the cavitation is absent at a low speed and the like, the pressure of the pump chamber facing the seal land portion may rapidly increase. In this state, however, the check valve may open the bypass path, thereby controlling an excessive pressure increase of the pump chamber at the seal land portion.
Japanese Patent Examined Publication No. 2638282 (equivalent of U.S. Pat. No. 5,096,397) discloses a trochoid type oil pump (referred to as xe2x80x9cSUCTION-CONTROLLED GEAR RING PUMPxe2x80x9d) having a function substantially similar to the oil pump under the above Japanese Patent Examined Publication No. 3074047 (=JP3074047B2, equivalent of Patent Number JP5099162).
In place of the one check valve disposed in the seal land portion in such a manner as to stride across the plurality of pump chambers under the Japanese Patent Examined Publication No. 3074047 (=JP3074047B2, equivalent of Patent Number JP5099162), the oil pump under Japanese Patent Examined Publication No. 2638282 (equivalent of U.S. Pat. No. 5,096,397) discloses a check valve which is provided for one of respective walls facing pump chambers. With a pressure of the pump chamber (facing the seal land portion) increased to or over a set pressure thereof at a low speed and the like, the check valve of each of the pump chambers can open a path individually, thereby conveying (relieving or releasing) the pressure of the pump chambers to a discharge port.
According to Japanese Patent Examined Publication No. 3074047 (=JP3074047B2, equivalent of Patent Number JP5099162), the valve body of the check valve strides across the plurality of the pump chambers and forms a part of a side wall of each of the pump chambers. The check valve when closed may allow the pump chambers to become substantially completely independent of each other, thus reducing the capacity. On the other hand, the check valve when opened may promptly (namely, substantially simultaneously with the opening) communicate the pump chambers to the discharge port""s side. In this case, however, the opening of the check valve is not necessarily be started at a point in time when the bubble in all the pump chambers is substantially completely smashed. In other words, as the case may be, the check valve may partly open when the bubble in one of the pump chambers (facing the valve body of the check valve) alone is substantially completely smashed, thereby rapidly conveying a high pressure of the discharge port to the other(s) of the pump chambers which contain(s) the remaining bubble caused by the cavitation, resulting in a rapid smashing of the bubble in the other(s) of the pump chambers. In sum, a failure caused by the cavitation cannot be reduced sufficiently.
On the other hand, Japanese Patent Examined Publication No. 2638282 (equivalent of U.S. Pat. No. 5,096,397) with each of the pump chambers having the check valve individually can be free from the above inconvenience. However, the check valve and the path which are to be so provided on a wall portion of a rotor as to correspond to each of the pump chambers may complicate the constitution and make production difficult. Thus, it is less likely to obtain a secured operation.
It is therefore an object of the present invention to provide an oil pump which can reduce a failure including an abnormal noise and the like which may be caused by a cavitation, with the oil pump simplified in constitution.
According to an aspect of the present invention, there is provided an oil pump comprising: 1) a plurality of pump chambers disposed substantially circumferentially and rotatable in accordance with a rotation of a drive shaft, a pump chamber of the plurality of the pump chambers defining a capacity which is substantially sequentially increased in a first certain angle range and substantially sequentially decreased in a second certain angle range, the pump chambers including; a) a first pump chamber, and b) a second pump chamber; 2) an intake port so formed as to be open in at least a part of an increase range where the pump chamber moves from a minimum capacity position to a maximum capacity position, the first pump chamber being free from being open to the intake port; 3) a discharge port so formed as to be open in at least a part of a decrease range where the pump chamber moves from the maximum capacity position to the minimum capacity position, in the decrease range the discharge port being biased to the minimum capacity position""s side, the second pump chamber being free from being open to the discharge port; 4) a seal land portion disposed in a certain section between: the maximum capacity position""s side of the intake port, and the maximum capacity position""s side of the discharge port, the seal land portion forming a stationary wall portion striding across the plurality of the pump chambers, the seal land portion sealing a communication between the intake port and the discharge port via the pump chamber; 5) a communication portion for allowing a mutual communication between the first pump chamber and the second pump chamber of the plurality of the pump chambers which face the seal land portion; 6) a reduced portion defined between the communication portion and the pump chamber; 7) a bypass path connecting the communication portion with the discharge port; and 8) a relief valve intervened in the bypass path and being adapted to open the bypass path with a pressure of the communication portion increased to or over a set pressure of the communication portion.
The other object(s) and feature(s) of the present invention will become understood from the following description with reference to the accompanying drawings.